It is necessary to find an improved test apparatus that allows soft recovery of high velocity projectiles with minimum breakage or fractures, and, in particular, an apparatus that facilitates accurate and inexpensive assessment of fragmentation characteristics and lethality of explosive fragmentation munitions upon detonation. Conventional techniques for recovery of high velocity Explosively Formed Fragments generated by Shaped Charge Jets, Explosively Formed Penetrator Warheads, and Explosive Fragmentation Munitions had been traditionally relying on low density fragment capture media such as packs of fiberboard (Celotex™) panels, ballistic gelatin, styrofoam, etc. In particular, in the case of Explosive Fragmentation Munitions, conventional recovery methods include firing munitions into an open-air arena fragmentation test structure (cylindrical structure enclosing the test warhead with a series of side-by-side packs of fiberboard panels) capturing only a small portion of the fragments, typically, bellow 6% of the total, or into completely enclosed rectangular or cylindrical plastic or steel structures filled with the fragment capture media, usually loose sawdust or water.
In a typical fragmentation arena test setup, tested munitions are positioned at the origin of the reference polar coordinate system and surrounded with series of velocity measuring screens and fragment-catching witness panels, all at significant distances from the warhead. The distance between the warhead and the fragment capture panels is determined by the weight of the explosive contained in the warhead, the type of the explosive, and the resilience of the panel structure to survive blast and fragment impact loads. For example, for an approximately 13 lb TNT-filled warhead, the standoff distances are in the order of approximately 6 meters, and by the time fragments reach the capture packs the average fragment velocities drop by approximately 20% to 30% compared to that at the burst. Defining the longitudinal axis of the munition as the polar axis z, the polar altitudinal angles Θ are measured from the munition's nose (Θ=0°) to the tail (Θ=180°), and the azimuthal angles φ are measured from an arbitrary projectile's feature (φ=0°) in a counterclockwise direction. In conventional fragmentation arena test procedures fragment sampling and fragment velocity measuring is usually limited to relatively small azimuthal sections, mainly because of enormous construction and data assessment costs associated with recovering fragments from the entire fragmenting shell. This sampling technique requires the assumption of isotropic fragmentation properties for all azimuthal angles φ throughout the entire Θ-angle zone (i.e., a complete altitudinal region bounded by two polar angles). By sampling small azimuthal angles across all polar zones from the munition nose to tail and adjusting this sample data mathematically, a prediction for entire munition fragment characterization is obtained. Since only a small azimuthal section of the region is sampled and scaled up, even relatively small munition positioning errors may result in large deviations of the fragmentation data, routinely requiring repeated testing for statistical data stability. For example, with only 6% of all the fragments recovered in the arena test structure, the discrepancy between numbers of fragments recovered from repeated fragmentation arena tests and that from a completely enclosed rectangular sawdust or water recovery structure is typically 25% to 30%.
In the second type of the fragmentation characterization test setup, commonly known as a “100%” fragment capture test, the tested munition is completely enclosed by the test structure filled with low density fragment capture media, usually sawdust or water, or with a combination thereof. Because the test structure can be re-used, and the fragment recovery and counting can be easily automated, the 100% fragment capture test costs are substantially lower than that of the fragmentation arena tests, making it extremely useful in the initial munition development phases.
In the “100%” fragment capture test, the fragment recovery rate is usually approximately 96% to 99.5% of the initial fragmentation case mass, depending on the test setup, the capture medium employed, and the mechanical properties of the fragmentation case material. Given the maximum 99.5% fragment recovery rate and typical fragmentation case materials, approximately 2% to 4% of the case mass usually ends up in a multitude of fragments with weights below 0.2-grains, partially because of the “natural” fragment formation process, partially because of the secondary fragment breakup caused by the capture medium. Since contribution to the lethality of small fragments with weight below 0.2-grains is minimal, these fragments are usually neglected in the fragmentation warhead characterization and commonly referred to as fragment “dust”.
In the case of the fragmentation arena test setup, because the fragments are first “gently” decelerated by the air drag and then captured by fiberboard or other types of capture panels, the secondary fragment breakage is usually significantly less than that in the 100% fragment recovery tests, either in the sawdust, or in the water. For example, X-ray flash-radiography of fragmentation munitions detonated in free air reveals presence of large numbers of long “spaghetti-like” fragments, never captured if the same warheads had been tested either in the water, or in the sawdust. It should be noted, that, although the “100%” fragment capture test data is a strong function of the fragment capture medium employed, the resulting data are usually statistically stable, with the average deviations in the order of 3% to 5%, at the maximum. Accordingly, the solution for improving the accuracy of the 100% fragment capture test can be found by optimizing properties of the fragment capture medium.
This invention relates to an improved fragment recovery test apparatus and, more particularly, to a test apparatus for “100%” recovery of fragments resulting from detonation of explosive fragmentation munitions. According to such means, a low density water-gas-bubble suspension medium is provided which enables optimal deceleration of high velocity fragments and minimization of fragment damage otherwise ordinarily exerted by the fragment capture medium drag forces. According to an embodiment of the invention, the munition is exploded into a water-gas-bubble mixture which is generated by pumping pressurized air through an air bubble dispenser immersed in a water tank; a test warhead is then loaded into a hollow openable plastic container supported within the water tank, and can then be detonated in such mixed water-gas-bubble suspension medium.